1. Technical Field of the Invention
The present invention relates to differential amplifiers and, in particular, to a differential amplifier including circuitry which improves slew rate.
2. Description of Related Art
Reference is made to FIG. 1 wherein there is shown a circuit diagram of a conventional and traditional, prior art, differential amplifier 10. The differential signal input Vi (having components Vi+ and Vi−) is received at the gate terminals of the differential input transistors (p-channel) M1 and M2. The source terminals of M1 and M2 are coupled together and to a first terminal of a current source 12 which supplies a current 21. A second terminal of the current source 12 is coupled to a reference voltage Vdd.
The drain terminal of M1 is coupled to the drain and gate terminals of (n-channel) transistor M4. A source terminal of transistor M4 is coupled to a reference voltage ground. The gate terminal of transistor M4 is further coupled to the gate of (n-channel) transistor M3. A source terminal of transistor M3 is also coupled to the reference voltage ground. The drain terminal of transistor M3 is coupled to the drain terminal and gate terminal of (p-channel) transistor M7. A source terminal of transistor M7 is coupled to the reference voltage Vdd.
The drain terminal of M2 is coupled to the drain and gate terminals of (n-channel) transistor M5. A source terminal of transistor M5 is coupled to the reference voltage ground. The gate terminal of transistor M5 is further coupled to the gate of (n-channel) transistor M6. A source terminal of transistor M6 is also coupled to the reference voltage ground. The drain terminal of transistor M6 is coupled to the drain terminal of (p-channel) transistor M8. A source terminal of transistor M8 is coupled to the reference voltage Vdd.
The gate terminals of transistors M7 and M8 are coupled together. Thus, current in transistor M1 is mirrored to transistor M8 using transistors M3, M4 and M7. The current of transistor M2 is mirrored to transistor M6 using transistor M5.
An output of the differential amplifier 10 is taken at the common drain terminal coupling between transistors M6 and M8. This point is labeled as node “OUT.” The output current Io1 from node OUT is the difference between the currents of transistors M8 and M6 (or in other words, the difference between the current of transistors M1 and M2 which receive the differential signal input Vi+ and Vi−).
A capacitor C is coupled between the output node OUT and the reference voltage ground. The capacitor C is known in the art as the stability compensation capacitor for closed loop operation.
When the differential input signal Vi is a positive large signal (the conclusion for a negative large signal being similar), transistor M1 is on and transistor M2 is off. In this condition, the currents in transistors M1 and M8 are equal to each other and to the 21 current supplied by current source 12. The current in transistors M2 and M6 are also equal to each other, but are zero. The output current Io1 is equal, as discussed above, to the difference between the currents of transistors M8 and M6, which in this case is the current 21 supplied by current source 12. By known equation, one can calculate the slew rate of the output voltage Vo at node OUT as:SR=Io1/C which is:SR=2I/C. 
Reference is now made to FIG. 2 which is a graph of voltage versus time with respect to the output voltage Vo of the traditional differential amplifier 10 of FIG. 1 when the input differential signal Vi a 2V peak-to-peak square wave (see, FIG. 3). The graph of FIG. 2 shows results when the differential amplifier 10 of FIG. 1 is configured as a unity gain buffer. The illustration is further made using a transient simulation analysis.
What can be seen in FIG. 2 is that the slew rate of the traditional differential amplifier 10 of FIG. 1, with respect to the input signal of FIG. 3, dictates a transition time period on the order of about 5 microseconds. In many known applications of differential amplifiers, this slew rate response is quite unacceptable (i.e., it is too slow). It can be improved by increasing the current source 12, but the power dissipation is also increased and the frequency response of the amplifier is changed and this may lead to instability.
A need accordingly exists in the art for a differential amplifier circuit which presents a faster and thus more acceptable slew rate than that provided by a traditional differential amplifier like that shown in FIG. 1.